A typical gas turbine engine includes a compressor, a combustor, and a turbine. Both the compressor and the turbine include alternating rows of rotating and stationary airfoils. Air flows axially through the engine. As is well known in the art, the compressed gases emerging from the compressor are mixed with fuel in the combustor and burned therein. The hot products of combustion, emerging from the combustor at high pressure, enter the turbine where the hot gases produce thrust to propel the engine and to drive the turbine which in turn drives the compressor.
The gas turbine engine operates in an extremely harsh environment characterized by vibrations and very high temperatures. The airfoils in the turbine are in jeopardy of burning because of the hot gases emerging from the combustor. Various cooling schemes exist to provide adequate cooling to these turbine airfoils. Many of these cooling schemes include intricate internal passages, such as a serpentine passage, that vent cooling air therethrough. The cooling schemes also include tiny cooling holes formed within the wall structure of the airfoils to allow the cooling air to pass therethrough.
The air that circulates through the airfoils, particularly during operation on the ground, includes particles of sand, dust, and other contaminants that have been ingested by the engine. The sand and dust, aided by extremely high temperatures and pressures, adhere to the surface of the internal cavity of the airfoils forming a crust, which may reduce the size or block entirely the air holes and the internal passages within the airfoil, thereby reducing the efficiency of the cooling thereof. To ensure that internal cavities are passable for the cooling air, the airfoils must be cleaned periodically during their lifetime or replaced. Since the airfoils are manufactured from expensive materials to withstand high temperatures, vibrations and cycling, frequent replacement of all the airfoils would be very costly. Therefore, cleaning of the airfoils is preferred. Furthermore, each engine includes hundreds of airfoils. Any reduction in time to clean each airfoil can potentially result in tremendous time savings and subsequently lead to significant cost savings.
A solution of VERSENE.RTM., the tetra-sodium salt of ethylenediamine tetra acetic acid EDTA, is a known cleaning solution in the aerospace industry. VERSENE, a registered trademark of Dow Chemical Company, acts as a metal chelating agent and is generally non-corrosive to the airfoils. However, the VERSENE solution has been known to be ineffective in terms of removing deposits from the internal cavities of airfoils. The VERSENE solution does not dissolve or remove the crust, but merely changes the characteristics of the crust in a chemical reaction.
Another known process for cleaning the internal cavities of the airfoils is an autoclave process. The autoclave process involves exposing the airfoils to high temperature and pressure fluid for a period of time. The process results in a loosening of the sand and dust layer. Following the autoclaving, a water blast at high pressure, directed at the internal cavity, removes the loosened layer of the sand and dust. Each airfoil must undergo many autoclave cycles to be effectively cleaned. Each cycle is time consuming and costly. Moreover, the autoclave process is effective in removing the crust only when the build-up is fine or the internal passage is not complicated. However, the method is not effective when the dust layer is thick or the passage is complicated.
The aerospace industry, in general, and overhaul and repair shops for the aerospace industry, in particular, are at loss as to how to effectively clean airfoils with intricate internal cooling passages. There is a potential for a great deal of cost savings on replacement airfoils if the cleaning process for the old airfoils is improved. Although the airfoil structure has become very sophisticated, the entire industry is searching for an improved method of cleaning the airfoils.